Gadd45 family of genes (Gadd45a, b, g) encode for cellular proteins rapidly induced by multiple stressors, including genotoxic & oncogenic stress. The unique role of Gadd45 proteins as sensors of oncogenes has been born out by several novel cancer mouse models in this laboratory, indicating that, dependent on the activated oncogene, gadd45 can function as either tumor promoter or suppressor by tethering distinct signaling pathways. Gadd45 has been identified as a mediator of oncogenic ras signaling. Ras mutations occur frequently in hematopoietic malignancies, including in AML, MPD and MDS. Recent evidence indicates that oncogenic N-RAS, K-RAS, and H- RAS exhibit different leukemogenic potentials in mice, suggesting that myeloid leukemogenesis by oncogenic RAS involves unique RAS signaling networks that need to be determined. On the other hand, BCR-ABL (BA) is known as the most common translocation in the myeloproliferative (MPD) disorder chronic myelogenous leukemia (CML) where an activated BA kinase oncoprotein impacts on cell proliferation and survival signaling pathways including Ras, PI3K, JAK-STAT, and PDk2-NFkB. The complex nature of these signaling pathways in the pathogenesis of CML is not fully understood. Recently Gadd45a expression was documented to be altered in a subset of AML patients. Our preliminary data indicate that Gadd45a behaves as an oncogene in context of N-RAS driven leukemia whereas both Gadd45a & b function as tumor suppressors in context of BA-driven leukemia. Also, Gadd45a,b expression was observed to be altered in human CML correlating with disease progression. The role of Gadd45 proteins as oncogenic stress sensors that modulate oncogene driven leukemias has not been studied, and understanding the role of these novel modulators in the molecular pathology of RAS and BCR-ABL is important. To this end, two Specific Aims are delineated: Aim 1 is targeted at assessing how Gaddd45a modulates RAS driven leukemogenicity. Sub-aim 1A will ascertain the effect of loss of Gadd45a on RAS-driven leukemic transformation in vivo~ Sub-aim 1B is targeted at studying the effect of loss of Gadd45a in BM expressing oncogenic RAS in vitro~ finally Sub-aim 1C will analyze human AML/MDS for activated RAS, alterations in Gadd45a and genes/pathways regulated by altered Gadd45a function. Aim 2 is targeted at exploring how Gaddd45a,b tumor suppressor functions impact on BA-driven leukemia and signaling. Sub-aim 2A will ascertain the effect of loss of Gadd45a & b function on BA-driven leukemia and Imatinib treatment~ Sub-aim 2B will explore how loss of Gadd45a and Gadd45b impact on BA oncogenic potential and imatinib treatment in vitro~ Sub-aim 2C will ascertain how gain of function of Gadd45a or b in primary BM impacts on BA oncogenic effect and imatinib treatment~ and, finally, Sub-aim 2D will analyze human CML for alterations in Gadd45a and Gadd45b and genes/pathways differentially regulated by altered function of Gaddd45a & Gadd45b. Knowledge gained from the proposed research should influence understanding of leukemogensis as well as add to the understanding of the role stress response genes play in other cancers, and contribute to the development of new/improved modalities for treatment of cancer.